Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-03-30
2003-06-24
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S306000, C324S309000, C324S312000
Reexamination Certificate
active
06583623
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to magnetic resonance imaging (MR imaging, or MRI) and more particularly to MR imaging in which water-only and fat-only images are taken without the interference of each other, and combined water-plus-fat images are formed.
DESCRIPTION OF RELATED ART
MR images are the presentation of the specific chemical environment of the imaged protons. The information from the chemical environment of the protons can be acquired in many different ways. A pulse sequence is applied to excite the protons and to acquire the information. The pulse sequence is designed to acquire specific information from specific chemical species. The most useful species in living tissues are water and fat. Therefore, several pulse sequences have been developed to acquire and display information only from water containing tissues or only from fat containing tissues. The routine sequence displays data from both water and fat containing tissues. The water-only images have widespread use in the clinical environment due to their ability to eliminate the high signal intensity fat and highlight the pathology in the organs and structures. The disadvantages in the previously developed sequences have been related to inadequate elimination of the chemical species not imaged (fat in particular), to a chemical shift artifact which is related to the different precession frequency between water and fat resulting in local misplacement of the anatomic structures, and to long acquisition time. In addition, if both fat suppressed (water-only) images and non-fat-suppressed (water-plus-fat) images are needed, two different data acquisitions are required with an increased imaging time. The long acquisition time to acquire water-plus-fat and water-only images is specifically a disadvantage in 2D dual-echo spin—echo (SE) imaging where acquisition times are inherently long.
Fat suppression sequences have often been used clinically to improve musculoskeletal system evaluation. Among those sequences, T2-weighted (T2W) water-only (fat-suppressed) spin echo images are most commonly used in clinical practice. Such images are described in T. T. Miller et al, “Fat-suppressed MRI of musculoskeletal infection: fast T2-weighted techniques versus gadolinium-enhanced T1-weighted techniques,” (Skeletal Radiol. 1997; 26:654-658). Although water-only images demonstrate the pathology, water-plus-fat (non-fat suppressed) proton density weighted (PDW) images are still needed for anatomic details. So far, water-only and water-plus-fat images are normally acquired using two separate imaging sequences. The increasing pressure in clinical practice to increase throughput, however, limits the number of sequences that can be prescribed in a clinical study, prompting the development of pulse sequences that will improve the ratio of information provided to the imaging time used.
Different techniques have been proposed to obtain water-only and fat-only images. In chemical saturation techniques an additional long spectrally selective RF pre-saturation pulse is applied before the RF excitation pulse to suppress the signal of the unwanted chemical species. The chemical saturation technique is explained by Harms S E, Flaming D P, Hesley K L, et al. “MR imaging of the breast with rotating delivery of excitation off resonance; clinical experience with pathologic correlation” (
Radiology
1993; 187:493-501); Haase A, Frahm J. “Multiple chemical shift selective NMR imaging using stimulated echoes” (
J Magn Reson
1985; 64:94-102); Pauly J M, Nishimura G G, Macovski A, “Multidimensional selective excitation” (
Proc Soc Magn Reson Med
1988;7:654); Joseph, P M, “A spin echo chemical shift MR imaging technique” (
J Comput Assist Tomogr
1985; 9:651-658); and Dumoulin C L, “A method for chemical-shift-selective imaging” (
Magn Reson Med
1985; 2:583-585). This technique, however, is sensitive to B
0
and B
1
inhomogeneities, and it also either increases the TR time or reduces the number of imaging slices.
Another method is the Dixon method, which is described in G. Glover et al, “Three-point Dixon technique for true fat/water decomposition with B
0
inhomogeneity correction,” (
Magn. Reson. Med
. 1995; 34:120-124). Although it provides water-only and fat-only images simultaneously and is less susceptible to the Bo inhomogeneity, it increases imaging time in gradient echo sequences. A modified Dixon method has been developed to reduce imaging time, as described by Lethimonnier F, Franconi F, Akoka S. “Three-point Dixon method with a MISSTEC sequence.” (
Magnetic Resonance Materials In Physics, Biology & Medicine
1997; 5(4): 285-288), but there is an accompanying loss in signal-to-noise ratio (SNR).
Recently, an echoplanar spectroscopic imaging technique that produces water, fat and water-plus-fat images without in-plane chemical-shift artifacts is described by Sarkar S, Heberlein K, Metzger G J, Zhang X D, Hu X P, “Applications of high-resolution echoplanar spectroscopic imaging for structural imaging.” (
J Magn Reson Imaging
1999;10:1-7). However, this technique suffers from poor SNR and cannot be used for high resolution imaging in clinical settings.
Another approach for simultaneous water and fat imaging uses spatial-spectral excitation with alternating water and fat acquisition schemes, and has been implemented on 2-D gradient echo (GRE) sequences, as described by Meyer C H, Pauly J M, Macovski A, Nishimura D G. “Simultaneous Spatial and Spectral Selective Excitation.” (
Magn Reson Med
1990; 15:287-304). Spatial-spectral excitation has also been applied to 2-D SE imaging, as described by Schick F. “Simultaneous highly selective MR water and fat imaging using a simple new type of spectral-spatial excitation.” (
Magn Reson Med
1998; 40:194-202) and has been found to provide better fat suppression than the conventional pre-saturation technique. Schick teaches 2D gradient echo imaging in which (water+fat) and (water−fat) signals are acquired. That technique requires at least two acquisitions and thus provides no saving in time compared to conventional fat and non-fat suppressed imaging. In-plane chemical shift correction by processing is not performed, and through-plane chemical shift misregistration cannot be corrected. Meyer et al teaches 2D gradient echo imaging in which water and fat signals are acquired alternately every TR/2 period. In-plane chemical shift is not mentioned. The maximum number of imaging slices is reduced relative to the regular 2D GRE sequence.
Recently, the present inventors developed a 3-D GRE pulse sequence that produced water images and fat images in a single acquisition time using spatial-spectral excitation. That study also provided a correction for the chemical shift artifacts that may hinder the evaluation of diseases such as osteonecrosis.
SUMMARY OF THE INVENTION
It is an object of the present invention to address all of the above issues.
It is another object of the invention to have water-only and fat-only SE images without interference of each other
It is still another object of the invention to acquire water-only and fat-only images during the same acquisition time, to acquire water-only and fat-only images, and to produce water-plus-fat images without chemical-shift artifact.
It is yet another object of the invention to develop a dual-echo spin echo (SE) sequence that simultaneously provides PDW water-only, PDW fat-only, PDW water-plus-fat images without chemical-shift artifact, and T2W water-only images in a single imaging time.
To achieve the above and other objects, the present invention is directed to a technique in a 2-D variable-echo dual-echo SE sequence which will be referred to as interleaved water and fat dual-echo spin echo imaging without chemical shift (IWFSEC). The technique can be implemented on commonly available equipment such as a 1.5 Tesla clinical scanner. By making efficient use of the timing between the first and second echoes, IWFSEC enables the simultaneous acquisition of PDW water-only, PDW fat-only and T2W water-only images in a single imagi
Kwok Wingchi Edmund
Totterman Saara Marjatta Sofia
Zhong Jianhui
Blank Rome LLP
Fetzner Tiffany A.
Lefkowitz Edward
University of Rochester
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